Advances in real-time phase-contrast flow MRI using asymmetric radial gradient echoes.

Purpose: To provide multidimensional velocity compensation for real-time phase-contrast flow MRI. Methods: The proposed method introduces asymmetric gradi- ent echoes for highly undersampled radial FLASH MRI with phase-sensitive image reconstruction by regularized nonlinear inversion (NLINV). Using an adapted gradient delay correction the resulting image quality was analyzed by simulations and experimentally validated at 3 Tesla. For real-time flow MRI the reduced gradient-echo timing allowed for the incorporation of velocity-compensating waveforms for all imaging gradients at even shorter repetition times. Results: The results reveal a usable degree of 20% asymme- try. Real-time flow MRI with full velocity compensation elimi- nated signal void in a flow phantom, confirmed flow parameters in healthy subjects and demonstrated signal recovery and phase conservation in a patient with aortic valve insufficiency and stenosis. Exemplary protocols at 1.4–1.5 mm resolution and 6 mm slice thickness achieved total acquisition times of 33.3–35.7 ms for two images (7 spokes each) with and without flow-encoding gradient. Conclusion: Asymmetric gradient echoes were successfully implemented for highly undersampled radial trajectories. The resulting temporal gain offers full velocity compensation for real-time phase-contrast flow MRI which minimizes false- positive contributions from complex flow and further enhances the temporal resolution compared with acquisitions with symmetric echoes